1. Field of the Invention
The present invention relates to a method of and a device for resolution conversion. More particularly, the invention concerns a method of and a device for resolution conversion, which, when performing a resolution conversion by using a DCT (orthogonal transform) technique, enable reducing block noises in a simple manner.
2. Description of the Related Art
Conventionally, as a method for resolution conversion (increasing/reduction an image), there have generally been used “nearest neighbor interpolation” (Nearest Neighbor), linear interpolation (Bi-Linear), cubic convolution interpolation (Cubic Convolution), etc. When increasing an image, pixels are interpolated using the interpolation technique, whereas, when reduction an image, pixels are thinned while they are being interpolated using the interpolation technique, or only thinning alone of pixels is performed. Each of these techniques performs up-sampling or down-sampling on discrete sampled data in the time/space domains, and uses an FIR (Finite Impulse Response) filter as low-pass filter processing for preventing the occurrence of return distortion. In these techniques using the FIR filter, it becomes a key how to use an FIR filter whose characteristic is near to an ideal low pass characteristic to the largest possible extent. In general, by increasing the number of taps of filter to thereby prepare a filter having a characteristic of f=Sin (πx)/(πx), i.e., a characteristic near to the so-called Sinc function, and to use the filter approximated with a finite length. Obviously, the larger the number of the taps is, the nearer to the Sinc function the characteristic is. However, the higher the cost becomes accordingly.
Also, further, there has also been proposed a resolution conversion method that is done not within the above-described time/space domain (temporal domain) but within the frequency domain. The resolution conversion method that is done within that frequency domain will now be explained using FIGS. 4A to 4C. For example, using orthogonal transform such as DCT (Discrete Cosine Transform), input image data is converted into frequency domain data per every block. FIG. 4A illustrates a case where the input data is converted every N×M (for example 8×8) size through the use of the DCT. FIG. 4B illustrates DCT coefficients of each block and, in this figure, a left upper corner pixel is a DC component, and, toward the right and bottom, the pixels become AC high-frequency components.
For increase, to the outside of the high-frequency domain components that are on the right and lower side of the block, there are added dummy high-frequency domain components (in the example of FIG. 4C, X pixels and Y pixels that respectively serve as dummy high-frequency domain components are added to the outside of the block of N×M to provide a block size of (N+X)×(M+Y)). For the every increased block, inverse orthogonal transform such as inverse DCT is performed to thereby convert again into the data within the time/space domain (temporal domain). On the other hand, in a case where decrease the image, conversely, the high-frequency domain components of the input signal are disposed away, and, for the every reduced block, inverse orthogonal transform such as inverse DCT is performed on the relevant data.
As the above-described resolution conversion method using orthogonal transform such as DCT, there have hitherto been proposed a method in which, as in, for example, Japanese Patent Application Laid-Open No. 2-76472, zero (0) are added as the dummy high-frequency domain components, a method in which, as in, for example, Japanese Patent Application Laid-Open No. 8-294001 that is mentioned below, frequency components obtained by transforming the input signal frequency components according to a prediction rule that has been prepared beforehand are added as dummy high-frequency domain components, etc.
However, in the above-described resolution conversion method using DCT, when increase, there occur the noises that are shaped like grating and that are called “block noises”.
As a method of reduction these block noises that occur when performing the resolution conversion in which DCT is used, there has been proposed a method in which, as in Japanese Patent Application Laid-Open No. 11-252356, DCT and inverse DCT are performed on the blocks in such a manner as the blocks are overlapped.
On the other hand, when compressing an MPEG (Moving Picture Experts Group)-2 used in a digital broadcasting, also, similarly, block noises occur. A lot of proposals have hitherto been made regarding the block noise reduction processing at the compressing time. However, when performing the block noise reduction processing with respect to every block border, blur occurs at the edge part of the image. Therefore, ordinarily, first, determination processing is performed with respect to between the edge part and the block noise part, then block noise reduction processing is performed. For example, a method wherein as in Japanese Patent Application Laid-Open No. Hei-2-57067 there is determined the value of difference between the border lines of the relevant two blocks; and, if the value of difference is small, it is determined that block noise is occurring, to thereby perform low-pass filter processing, or a method wherein as in Japanese Patent Application Laid-Open No. 2000-299859 there is detected the block containing only the high-frequency domain components the quantity of which is not larger than a relevant threshold value, whereby block noise reduction processing is performed with respect to only the block alone, or etc. has hitherto been proposed.